The prior art is illustrated for example by patents EP-40,161, EP-194,450 or EP-309,596, which describe pumps for applications to liquid chromatography, and embodiments wherein a pump can be associated with a system of solenoid valves to obtain elution gradients with several solvents.
Patent EP-0,709,572 (U.S. Pat. No. 5,755,561) filed by the applicants also describes a pumping system comprising one or more pumping modules connected by one-way valves respectively to vessels containing fluids to be mixed, a collection head receiving the fluids delivered by the various primary modules, and a secondary pumping unit with a piston for discharging at a substantially constant rate the mixture of fluids collected in this head. The pistons are alternately displaced either through rotation of the cams or through translation of the endless screws. A control unit associated with position detectors and pressure detectors proportions the fluids injected and adjusts the phases, the piston strokes and their speed so as to obtain a substantially constant discharge rate. Each primary module can also comprise a second pumping module allowing to obtain also a constant suction rate.
Patent FR-2,768,189 (U.S. Pat. No. 6,116,869) also filed by the applicants describes another pumping system allowing to obtain a mixture of liquids with well-adjusted proportions and flow rates. It comprises a liquid mixing device located upstream from a pump. The liquids are taken from vessels and cyclically fed, in predetermined proportions, into a mixing chamber by alternate opening of solenoid valves working under on-off conditions. The suction phase of the system is controlled by using damping means such as bellows in antechambers so as to avoid the effects of velocity discontinuities when the valves open and close. The flow rate of the pump is controlled upon suction as well as upon discharge.
An example of a reciprocating pumping unit that can be used is diagrammatically shown in FIGS. 1, 2. It comprises a pump barrel 1 provided with a cylindrical inner cavity 2. Through an opening in the bottom of the barrel, a rod 3 partially fits into inner cavity 2. Sealing means 4 arranged around the rod insulate the inner cavity. Rod 3 is provided with a head 5. A spring 6 is arranged between it and the end of the barrel so as to exert a permanent extraction force on the piston.
At the opposite end thereof, inner cavity 2 communicates with a line 7 provided with a one-way valve 8 (such as a ball check valve) which opens during the suction phase as rod 3 moves back. A pressure detector C is arranged for example in line 7 downstream from valve 8.
According to a first embodiment (FIG. 1), the more or less great depth of penetration of rod 3 in inner cavity 2 is provided by the rotation of a cam 9 resting against head 5, whose shaft 10 is driven in rotation by a motor 11. The more or less great depth of penetration of rod 3 in cylindrical cavity 2 is obtained by changing the eccentricity d of the cam on the shaft.
According to a second embodiment (FIG. 2), the more or less great depth of penetration of rod 3 in inner cavity 2 is obtained by the translation of an endless screw 12 resting on head 5 by means of a ball thrust 13. The screw translation means include for example a nut 14 suited to screw 12, which is for example housed in the hollow rotor of a stationary electric motor 15 which drives it into rotation. The direction of translation of the screw is changed by inverting the direction of rotation of the motor every pumping half cycle. Motors 11 or 15 can be, for example, direct-current or stepping type motors.
Line 7 upstream from non-return valve 8 is connected by means of solenoid valves EV1, EV2, EVn respectively to vessels containing the fluid components A, B, . . . , X to be mixed. An associated control unit PC adjusts the opening of the solenoid valves while observing predetermined ratios between their respective opening times according to the desired concentration ratios between the solvents injected.
Experience shows that this proportioning mode is very imprecise. The real proportion of the constituents in the mixture sucked is far from corresponding to the proportion that would be expected when relying on the respective opening times of the solenoid valves.
After studying this problem, it has been noticed that the non-return valves were the main cause of this imprecision because, in practice, their opening and closing times are different. If the opening order of the solenoid valves during a suction phase is of the order EV1, EV2, . . . , EVn, for example, the effective proportions of the first constituent injected A and of the last constituent injected X are differently affected by the unequal opening and closing times of ball check valve 8.